Pixel architecture and driving scheme for biometric sensing

ABSTRACT

Embodiments herein describe an input device that includes a rectangular array of sensor electrodes connected to sensor modules that measure capacitive sensing signals corresponding to the electrodes. When performing code division multiplexing (CDM), multiple sensor electrodes are coupled to the same sensor module. As such, the sensor module generates a measurement that represents the sum of the charges on the sensor electrodes rather than an individual charge on a single sensor electrode. By repeatedly sensing multiple sensor electrodes in parallel, the input device can determine a change of capacitance for each individual sensor electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 15/406,109, filed Jan. 13, 2017. The aforementionedrelated patent application is herein incorporated by reference in itsentirety.

FIELD OF THE INVENTION

This invention generally relates to electronic devices and performingcapacitive sensing.

BACKGROUND

Many input devices include a fingerprint sensor that uses capacitivesensing to detect a fingerprint of a user. A fingerprint sensortypically includes a sensing region in which the fingerprint sensordetermines the ridges and valley of a finger. In one example, thesensing region includes sensor electrodes used to measure changes incapacitance resulting from a finger interacting with the sensing region.However, common mode coupling due to capacitance between a sensorelectrode and neighboring sensor electrodes can interfere with measuringcapacitance values between the sensor electrode and the finger which isespecially the case with fingerprint sensors which measure signals withsmall magnitudes. In addition, the input device may have parasiticcapacitances corresponding to an output line used to drive signals onthe sensor electrode which can be orders of magnitude larger than thecapacitance between the sensor electrode and the finger. The effects ofthe common mode coupling and the parasitic capacitance make measuringthe smaller capacitance between the sensing electrode and the fingermore difficult.

BRIEF SUMMARY OF THE INVENTION

One embodiment described herein includes a processing system foroperating an input device comprising a plurality of sensor electrodesdisposed in an array, wherein a first sensor electrode and a secondsensor electrode of the plurality of sensor electrodes are disposed in asame column in the array. The processing system includes a controllercircuit configured to charge the first sensor electrode to a firstvoltage, charge the second sensor electrode to a second voltagedifferent from the first voltage, wherein the first and second voltagesare assigned to the first and second sensor electrodes according to acode division multiplexing (CDM) technique, and, after charging thefirst and second sensor electrodes, couple the first and second sensorelectrodes simultaneously to a same analog front end (AFE) to sense acombined charge on the first and second sensor electrodes.

Another embodiment described herein includes an input device thatincludes a plurality of sensor electrodes disposed in columns forming anarray where each column comprises a sense line coupled to an AFE andrespective transistors selectively coupling the plurality of sensorelectrodes in the column to the sense line and a processing system. Theprocessing system is configured to charge at least one of the pluralityof sensor electrodes in at least one of the columns to a first voltageand after charging the at least one of the plurality of sensorelectrodes: couple the at least one of the plurality of sensorelectrodes to the sense line by activating one of the respectivetransistors, deactivate an offset transistor coupled to the sense lineto mitigate a charge injected by activating the one of the respectivetransistors, and measure a charge on the at least one of the pluralityof sensor electrodes using the AFE after deactivating the offsettransistor.

Another embodiment described herein is a processing system for operatingan input device comprising a plurality of sensor electrodes disposed incolumns forming an array where each column comprises a sense linecoupled to an AFE and respective transistors selectively coupling theplurality of sensor electrodes in the column to the sense line. Theprocessing system includes a controller configured to charge at leastone of the plurality of sensor electrodes in at least one of the columnsto a first voltage and after charging the at least one of the pluralityof sensor electrodes: couple the at least one of the plurality of sensorelectrodes to the sense line by activating one of the respectivetransistors and deactivate an offset transistor coupled to the senseline to mitigate a charge injected by activating the one of therespective transistors. The processing system includes a sensor modulecomprising the AFE, the sensor module is configured to measure a chargeon the at least one of the plurality of sensor electrodes using the AFEafter the offset transistor is deactivated.

Another embodiment described herein includes an input device thatincludes a plurality of sensor electrodes disposed in columns forming anarray, wherein each column comprises a sense line coupled to an AFE andrespective transistors selectively coupling the plurality of sensorelectrodes in the column to the sense line. The AFE is configured tocharge a first sensor electrode in at least one of the columns to afirst voltage, charge a second sensor electrode in the column to asecond voltage different from the first voltage, and, after charging thefirst and second sensor electrodes, measure a combined charge on thefirst and second sensor electrodes in parallel.

BRIEF DESCRIPTION OF DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this disclosure and are therefore not to beconsidered limiting of its scope, for the disclosure may admit to otherequally effective embodiments.

FIG. 1 is a block diagram of an exemplary system that includes an inputdevice in accordance with an embodiment of the invention;

FIG. 2 is input device that includes a matrix sensor arrangement inaccordance with an embodiment of the invention;

FIG. 3 illustrates a sensor layout for detecting an input object inaccordance with an embodiment of the invention;

FIG. 4 illustrates a sensor module for detecting an input object inaccordance with an embodiment of the invention;

FIG. 5 is a flowchart for controlling neighboring electrodes whenperforming capacitive sensing in accordance with an embodiment of theinvention;

FIGS. 6A-6C illustrate sensing patterns for operating sensor electrodesin accordance with an embodiment of the invention;

FIG. 7 is a timing diagram corresponding to the sensor layout shown inFIG. 6A in accordance with an embodiment of the invention;

FIGS. 8A and 8B illustrates a sensor layout for detecting an inputobject in accordance with an embodiment of the invention;

FIG. 9 illustrates a timing chart for using offset sensor electrodeswhen performing capacitive sensing in accordance with an embodiment ofthe invention;

FIGS. 10A-10E illustrate operational states of a column of sensorelectrodes during various time periods illustrated in FIG. 9 inaccordance with embodiments of the invention;

FIG. 11 illustrates a timing chart for using offset sensor electrodeswhen performing capacitive sensing in accordance with an embodiment ofthe invention;

FIG. 12 illustrates a sensor module in accordance with an embodiment ofthe invention; and

FIG. 13 illustrates a sensor module in accordance with an embodiment ofthe invention.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation. The drawings referred to here should not beunderstood as being drawn to scale unless specifically noted. Also, thedrawings are often simplified and details or components omitted forclarity of presentation and explanation. The drawings and discussionserve to explain principles discussed below, where like designationsdenote like elements.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following detailed description.

Various embodiments of the present invention provide input devices andmethods that facilitate improved usability. In one embodiment, the inputdevice includes a matrix sensor which, as defined herein, includes aplurality of sensor electrodes arranged in an array on a common surfaceor plane. The input device may include a plurality of sensor modulesconnected to the sensor electrodes via switches that measure capacitivesensing signals corresponding to the electrodes. During a charge stage,the input device applies a charging voltage to at least one of theelectrodes in the matrix sensor. If a code division multiplexing (CDM)scheme is used, the input device applies the charging voltage to aplurality of sensor electrodes connected to the same sense line. Theamount of charge accumulated on the selected sensor electrode (orelectrodes) depends on the capacitive coupling between the sensorelectrode and an input object (e.g., a finger). During a read stage, theinput device measures the amount of charge accumulated on the one ormore sensor electrodes during the charge stage. In one embodiment, themeasured charge can be correlated to a particular feature of an inputobject. For example, when used as a fingerprint sensor, the input devicecan detect valleys and ridges in a finger depending on the measuredcharge.

The capacitive coupling between a selected sensor electrode and theinput object, however, is not the only capacitance that can affect theamount of charge stored on the sensor electrode during the charge stage.The capacitive coupling between the selected sensor electrode and theneighboring sensor electrodes in the matrix sensor when used as afingerprint sensor can be on the same order of magnitude as thedifference of the capacitive coupling between a ridge in the finger tothe selected electrode and the capacitive coupling between a valley inthe finger to the selected electrode. Because this capacitive couplingcan make measuring the charge attributable to the capacitive coupling tothe input object more difficult, embodiments herein drive theneighboring electrodes in the same manner as the selected sensorelectrode or electrodes. In one embodiment, during the charge stage, theselected electrode (also referred to herein as the “sensed electrode”)and the neighboring electrodes are connected to the charge voltage.Because there is no voltage difference between these electrodes, thecapacitive coupling to the neighboring electrodes does not affect theamount of charge stored on the selected electrode during the chargestage. Similarly, during the read stage, the neighboring electrodes andthe selected electrode may be biased to the same reference voltage—e.g.,ground—so there is again no voltage difference between the electrodes.In this manner, the effects of the parasitic capacitance between theselected sensor electrode and its neighbors can be mitigated or removed.

Turning now to the figures, FIG. 1 is a block diagram of an exemplaryinput device 100, in accordance with embodiments of the invention. Theinput device 100 may be configured to provide input to an electronicsystem (not shown). As used in this document, the term “electronicsystem” (or “electronic device”) broadly refers to any system capable ofelectronically processing information. Some non-limiting examples ofelectronic systems include personal computers of all sizes and shapes,such as desktop computers, laptop computers, netbook computers, tablets,web browsers, e-book readers, and personal digital assistants (PDAs).Additional example electronic systems include composite input devices,such as physical keyboards that include input device 100 and separatejoysticks or key switches. Further example electronic systems includeperipherals such as data input devices (including remote controls andmice), and data output devices (including display screens and printers).Other examples include remote terminals, kiosks, and video game machines(e.g., video game consoles, portable gaming devices, and the like).Other examples include communication devices (including cellular phones,such as smart phones), and media devices (including recorders, editors,and players such as televisions, set-top boxes, music players, digitalphoto frames, and digital cameras). Additionally, the electronic systemcould be a host or a slave to the input device.

The input device 100 can be implemented as a physical part of theelectronic system, or can be physically separate from the electronicsystem. As appropriate, the input device 100 may communicate with partsof the electronic system using any one or more of the following: buses,networks, and other wired or wireless interconnections. Examples includeI²C, SPI, PS/2, Universal Serial Bus (USB), Bluetooth, RF, and IRDA.

In FIG. 1, the input device 100 is shown as a proximity sensor device(also often referred to as a “touchpad” or a “touch sensor device”)configured to sense input provided by one or more input objects 140 in asensing region 120. Example input objects include fingers and styli, asshown in FIG. 1. In one embodiment, the input device 100 is afingerprint sensor that senses the different features in a finger suchas ridges and valleys which can be used to form a fingerprint. Thefingerprint sensor may be a swipe sensor, where a fingerprint image isreconstructed from a series of scans as the user moves their finger overthe sensor, or a placement sensor, where a sufficient area of thefingerprint can be captured from a single scan as the user holds herfinger at a fixed location in the sensing region 120.

Sensing region 120 encompasses any space above, around, in and/or nearthe input device 100 in which the input device 100 is able to detectuser input (e.g., user input provided by one or more input objects 140).The sizes, shapes, and locations of particular sensing regions may varywidely from embodiment to embodiment. In some embodiments, the sensingregion 120 extends from a surface of the input device 100 in one or moredirections into space until signal-to-noise ratios prevent sufficientlyaccurate object detection. The distance to which this sensing region 120extends in a particular direction, in various embodiments, may be on theorder of less than a millimeter, millimeters, centimeters, or more, andmay vary significantly with the type of sensing technology used and theaccuracy desired. Thus, some embodiments sense input that comprises nocontact with any surfaces of the input device 100, contact with an inputsurface (e.g. a touch surface) of the input device 100, contact with aninput surface of the input device 100 coupled with some amount ofapplied force or pressure, and/or a combination thereof. In variousembodiments, input surfaces may be provided by surfaces of casingswithin which the sensor electrodes reside, by face sheets applied overthe sensor electrodes or any casings, etc. In some embodiments, thesensing region 120 has a rectangular shape when projected onto an inputsurface of the input device 100. In another embodiment, the sensingregion 120 has a circular shape that conforms to the shape of afingertip.

The input device 100 may utilize any combination of sensor componentsand sensing technologies to detect user input in the sensing region 120.The input device 100 comprises one or more sensing elements fordetecting user input.

Some implementations are configured to provide images that span one,two, three, or higher dimensional spaces. Some implementations areconfigured to provide projections of input along particular axes orplanes.

In some capacitive implementations of the input device 100, voltage orcurrent is applied to create an electric field. Nearby input objectscause changes in the electric field, and produce detectable changes incapacitive coupling that may be detected as changes in voltage, current,or the like.

Some capacitive implementations utilize arrays or other regular orirregular patterns of capacitive sensing elements to create electricfields. In some capacitive implementations, separate sensing elementsmay be ohmically shorted together to form larger sensor electrodes. Somecapacitive implementations utilize resistive sheets, which may beuniformly resistive.

Some capacitive implementations utilize “self capacitance” (or “absolutecapacitance”) sensing methods based on changes in the capacitivecoupling between sensor electrodes and an input object. In variousembodiments, an input object near the sensor electrodes alters theelectric field near the sensor electrodes, thus changing the measuredcapacitive coupling. In one implementation, an absolute capacitancesensing method operates by modulating sensor electrodes with respect toa reference voltage (e.g. system ground), and by detecting thecapacitive coupling between the sensor electrodes and input objects.

Some capacitive implementations utilize “mutual capacitance” (or“transcapacitance”) sensing methods based on changes in the capacitivecoupling between sensor electrodes. In various embodiments, an inputobject near the sensor electrodes alters the electric field between thesensor electrodes, thus changing the measured capacitive coupling. Inone implementation, a transcapacitive sensing method operates bydetecting the capacitive coupling between one or more transmitter sensorelectrodes (also “transmitter electrodes” or “transmitters”) and one ormore receiver sensor electrodes (also “receiver electrodes” or“receivers”). Transmitter sensor electrodes may be modulated relative toa reference voltage (e.g., system ground) to transmit transmittersignals. Receiver sensor electrodes may be held substantially constantrelative to the reference voltage to facilitate receipt of resultingsignals. A resulting signal may comprise effect(s) corresponding to oneor more transmitter signals, and/or to one or more sources ofenvironmental interference (e.g. other electromagnetic signals). Sensorelectrodes may be dedicated transmitters or receivers, or may beconfigured to both transmit and receive.

In FIG. 1, a processing system 110 is shown as part of the input device100. The processing system 110 is configured to operate the hardware ofthe input device 100 to detect input in the sensing region 120. Theprocessing system 110 comprises parts of or all of one or moreintegrated circuits (ICs) and/or other circuitry components. Forexample, a processing system for a mutual capacitance sensor device maycomprise transmitter circuitry configured to transmit signals withtransmitter sensor electrodes, and/or receiver circuitry configured toreceive signals with receiver sensor electrodes). In some embodiments,the processing system 110 also comprises electronically-readableinstructions, such as firmware code, software code, and/or the like. Insome embodiments, components composing the processing system 110 arelocated together, such as near sensing element(s) of the input device100. In other embodiments, components of processing system 110 arephysically separate with one or more components close to sensingelement(s) of input device 100, and one or more components elsewhere.For example, the input device 100 may be a peripheral coupled to adesktop computer, and the processing system 110 may comprise softwareconfigured to run on a central processing unit of the desktop computerand one or more ICs (perhaps with associated firmware) separate from thecentral processing unit. As another example, the input device 100 may bephysically integrated in a phone, and the processing system 110 maycomprise circuits and firmware that are part of a main processor of thephone. In some embodiments, the processing system 110 is dedicated toimplementing the input device 100. In other embodiments, the processingsystem 110 also performs other functions, such as operating displayscreens, driving haptic actuators, etc.

The processing system 110 may be implemented as a set of modules thathandle different functions of the processing system 110. Each module maycomprise circuitry that is a part of the processing system 110,firmware, software, or a combination thereof. In various embodiments,different combinations of modules may be used. Example modules includehardware operation modules for operating hardware such as sensorelectrodes and display screens, data processing modules for processingdata such as sensor signals and positional information, and reportingmodules for reporting information. Further example modules includesensor operation modules configured to operate sensing element(s) todetect input, identification modules configured to identify gesturessuch as mode changing gestures, and mode changing modules for changingoperation modes.

In some embodiments, the processing system 110 responds to user input(or lack of user input) in the sensing region 120 directly by causingone or more actions. Example actions include changing operation modes(e.g., unlocking the user device or providing access to secure datausing a detected fingerprint), as well as GUI actions such as cursormovement, selection, menu navigation, and other functions. In someembodiments, the processing system 110 provides information about theinput (or lack of input) to some part of the electronic system (e.g. toa central processing system of the electronic system that is separatefrom the processing system 110, if such a separate central processingsystem exists). In some embodiments, some part of the electronic systemprocesses information received from the processing system 110 to act onuser input, such as to facilitate a full range of actions, includingmode changing actions and GUI actions.

For example, in some embodiments, the processing system 110 operates thesensing element(s) of the input device 100 to produce electrical signalsindicative of input (or lack of input) in the sensing region 120. Theprocessing system 110 may perform any appropriate amount of processingon the electrical signals in producing the information provided to theelectronic system. For example, the processing system 110 may digitizeanalog electrical signals obtained from the sensor electrodes. Asanother example, the processing system 110 may perform filtering orother signal conditioning. As yet another example, the processing system110 may subtract or otherwise account for a baseline, such that theinformation reflects a difference between the electrical signals and thebaseline. As yet further examples, the processing system 110 maydetermine positional information, recognize inputs as commands,recognize handwriting, and the like.

“Positional information” as used herein broadly encompasses absoluteposition, relative position, velocity, acceleration, and other types ofspatial information. Exemplary “zero-dimensional” positional informationincludes near/far or contact/no contact information. Exemplary“one-dimensional” positional information includes positions along anaxis. Exemplary “two-dimensional” positional information includesmotions in a plane. Exemplary “three-dimensional” positional informationincludes instantaneous or average velocities in space. Further examplesinclude other representations of spatial information. Historical dataregarding one or more types of positional information may also bedetermined and/or stored, including, for example, historical data thattracks position, motion, or instantaneous velocity over time.

In some embodiments, the input device 100 is implemented with additionalinput components that are operated by the processing system 110 or bysome other processing system. These additional input components mayprovide redundant functionality for input in the sensing region 120, orsome other functionality. FIG. 1 shows buttons 130 near the sensingregion 120 that can be used to facilitate selection of items using theinput device 100. Other types of additional input components includesliders, balls, wheels, switches, and the like. Conversely, in someembodiments, the input device 100 may be implemented with no other inputcomponents.

In some embodiments, the input device 100 comprises a touch screeninterface, and the sensing region 120 overlaps at least part of anactive area of a display screen. For example, the input device 100 maycomprise substantially transparent sensor electrodes overlaying thedisplay screen and provide a touch screen interface for the associatedelectronic system. The display screen may be any type of dynamic displaycapable of displaying a visual interface to a user, and may include anytype of light emitting diode (LED), organic LED (OLED), cathode ray tube(CRT), liquid crystal display (LCD), plasma, electroluminescence (EL),or other display technology. The input device 100 and the display screenmay share physical elements. For example, some embodiments may utilizesome of the same electrical components for displaying and sensing. Asanother example, the display screen may be operated in part or in totalby the processing system 110.

It should be understood that while many embodiments of the invention aredescribed in the context of a fully functioning apparatus, themechanisms of the present invention are capable of being distributed asa program product (e.g., software) in a variety of forms. For example,the mechanisms of the present invention may be implemented anddistributed as a software program on information bearing media that arereadable by electronic processors (e.g., non-transitorycomputer-readable and/or recordable/writable information bearing mediareadable by the processing system 110). Additionally, the embodiments ofthe present invention apply equally regardless of the particular type ofmedium used to carry out the distribution. Examples of non-transitory,electronically readable media include various discs, memory sticks,memory cards, memory modules, and the like. Electronically readablemedia may be based on flash, optical, magnetic, holographic, or anyother storage technology.

FIG. 2 shows a portion of an exemplary pattern of capacitive sensingpixels 205 (also referred to herein as capacitive pixels or sensingpixels) configured to sense in the sensing region 120 associated with apattern, according to some embodiments. Each capacitive pixel 205 mayinclude one of more of the sensing elements described above. For clarityof illustration and description, FIG. 2 presents the regions of thecapacitive pixels 205 in a pattern of simple rectangles and does notshow various other components within the capacitive pixels 205. In oneembodiment, the capacitive sensing pixels 205 are areas of localizedcapacitance (capacitive coupling). Capacitive pixels 205 may be formedbetween an individual sensor electrode and ground in a first mode ofoperation and between groups of sensor electrodes used as transmitterand receiver electrodes in a second mode of operation. The capacitivecoupling changes with the proximity and motion of input objects in thesensing region 120 associated with the capacitive pixels 205, and thusmay be used as an indicator of the presence of the input object in thesensing region 120 of the input device or to detect ridges and valleyswhen used as a fingerprint sensor.

The exemplary pattern comprises an array of capacitive sensing pixels205X,Y (referred collectively as pixels 205) arranged in X columns and Yrows in a common plane, wherein X and Y are positive integers, althoughone of X and Y may be zero. It is contemplated that the pattern ofsensing pixels 205 may comprises a plurality of sensing pixels 205having other configurations, such as polar arrays, repeating patterns,non-repeating patterns, non-uniform arrays a single row or column, orother suitable arrangement. Further, as will be discussed in more detailbelow, the sensor electrodes in the sensing pixels 205 may be any shapesuch as circular, rectangular, diamond, star, square, noncovex, convex,nonconcave concave, etc. As shown here, the sensing pixels 205 arecoupled to the processing system 110.

In a first mode of operation, at least one sensor electrode within thecapacitive sensing pixels 205 may be utilized to detect the presence ofan input object via absolute sensing techniques. A sensor module 204(e.g., a sensor circuit) in processing system 110 is configured to drivea sensor electrode using a trace 240 in each pixel 205 with a capacitivesensing signal (which can be modulated or unmodulated) and measure acapacitance between the sensor electrode, and the input object (e.g.,free space or earth ground) based on the capacitive sensing signal,which is utilized by the processing system 110 or other processor todetermine the position of the input object or features in a finger.

The various electrodes of capacitive pixels 205 are typically ohmicallyisolated from the electrodes of other capacitive pixels 205.Additionally, where a pixel 205 includes multiple electrodes, theelectrodes may be ohmically isolated from each other. That is, one ormore insulators separate the sensor electrodes and prevent them fromelectrically shorting to each other.

In a second mode of operation, sensor electrodes in the capacitivepixels 205 are utilized to detect the presence of an input object viatranscapacitance sensing techniques. That is, processing system 110 maydrive at least one sensor electrode in a pixel 205 with a transmittersignal and receive resulting signals using one or more of the othersensor electrodes in the pixel 205, where a resulting signal comprisingeffects corresponding to the transmitter signal. The resulting signal isutilized by the processing system 110 or other processor to determinethe position of the input object.

The input device 100 may be configured to operate in any one of themodes described above. The input device 100 may also be configured toswitch between any two or more of the modes described above.

In some embodiments, the capacitive pixels 205 are “scanned” todetermine these capacitive couplings. That is, in one embodiment, one ormore of the sensor electrodes are driven to transmit transmittersignals. Transmitters may be operated such that one transmitterelectrode transmits at one time, or multiple transmitter electrodestransmit at the same time. Where multiple transmitter electrodestransmit simultaneously, the multiple transmitter electrodes maytransmit the same transmitter signal and effectively produce aneffectively larger transmitter electrode. Alternatively, the multipletransmitter electrodes may transmit different transmitter signals. Forexample, multiple transmitter electrodes may transmit differenttransmitter signals according to one or more coding schemes that enabletheir combined effects on the resulting signals of receiver electrodesto be independently determined.

The sensor electrodes configured as receiver sensor electrodes may beoperated singly or multiply to acquire resulting signals. The resultingsignals may be used to determine measurements of the capacitivecouplings at the capacitive pixels 205.

In other embodiments, “scanning” pixels 205 to determine thesecapacitive coupling includes driving with a modulated signal andmeasuring the absolute capacitance of one or more of the sensorelectrodes. In another embodiment, the sensor electrodes may be operatedsuch that the modulated signal is driven on a sensor electrode inmultiple capacitive pixels 205 at the same time. In such embodiments, anabsolute capacitive measurement may be obtained from each of the one ormore pixels 205 simultaneously. In one embodiment, the input device 100simultaneously drives a sensor electrode in a plurality of capacitivepixels 205 and measures an absolute capacitive measurement for each ofthe pixels 205 in the same sensing cycle. In various embodiments,processing system 110 may be configured to selectively drive and receivewith a portion of sensor electrodes. For example, the sensor electrodesmay be selected based on, but not limited to, an application running onthe host processor, a status of the input device, an operating mode ofthe sensing device and a determined location of an input object. Inanother embodiment, the input object (e.g., a finger) is the transmitterthat is driven with the modulated signal while the sensor electrode is areceiver.

A set of measurements from the capacitive pixels 205 form a capacitiveimage (also capacitive frame) representative of the capacitive couplingsat the pixels 205 as discussed above. Multiple capacitive images may beacquired over multiple time periods, and differences between them usedto derive information about input in the sensing region. For example,successive capacitive images acquired over successive periods of timecan be used to track the motion(s) of one or more input objectsentering, exiting, and within the sensing region.

In some embodiments, one or more of the sensor electrodes in thecapacitive pixels 205 include one or more display electrodes used inupdating the display of the display screen. In one or more embodiments,the display electrodes comprise one or more segments of a Vcom electrode(common electrodes), a source drive line, gate line, an anode electrodeor cathode electrode, or any other display element. These displayelectrodes may be disposed on an appropriate display screen substrate.For example, the electrodes may be disposed on the a transparentsubstrate (a glass substrate, TFT glass, a plastic substrate or anyother transparent material) in some display screens (e.g., In PlaneSwitching (IPS) or Plane to Line Switching (PLS) Organic Light EmittingDiode (OLED)), on the bottom of the color filter glass of some displayscreens (e.g., Patterned Vertical Alignment (PVA) or Multi-domainVertical Alignment (MVA)), over an emissive layer (OLED), etc. In suchembodiments, an electrode that is used as both a sensor and a displayelectrode can also be referred to as a combination electrode, since itperforms multiple functions.

Continuing to refer to FIG. 2, the processing system 110 coupled to thesensing electrodes includes a sensor module 204 and optionally, adisplay driver module 208. In one embodiment the sensor module comprisescircuitry configured to drive a transmitter signal onto and receiveresulting signals with the resulting signals the sensing electrodesduring periods in which input sensing is desired. In one embodiment thesensor module 204 includes a transmitter module including circuitryconfigured to drive a transmitter signal onto the sensing electrodesduring periods in which input sensing is desired. The transmitter signalis generally modulated and contains one or more bursts over a period oftime allocated for input sensing. The transmitter signal may have anamplitude, frequency and voltage which may be changed to obtain morerobust location information of the input object in the sensing region.The modulated signal used in absolute capacitive sensing may be the sameor different from the transmitter signal used in transcapacitancesensing. The sensor module 204 may be selectively coupled to one or moreof the sensor electrodes in the capacitive pixels 205. For example, thesensor module 204 may be coupled to selected portions of the sensorelectrodes and operate in either an absolute or transcapacitance sensingmode. In another example, the sensor module 204 may be coupled todifferent sensor electrodes when operating in the absolute sensing modethan when operating in the transcapacitance sensing mode.

In various embodiments the sensor module 204 may comprise a receivermodule that includes circuitry configured to receive a resulting signalwith the sensing electrodes comprising effects corresponding to thetransmitter signal during periods in which input sensing is desired. Inone or more embodiments, the receiver module is configured to drive amodulated signal onto a first sensor electrode in one of the pixels 205and receive a resulting signal corresponding to the modulated signal todetermine changes in absolute capacitance of the sensor electrode. Thereceiver module may determine a position of the input object in thesensing region 120 or may provide a signal including informationindicative of the resulting signal to another module or processor, forexample, a determination module or a processor of the electronic device(e.g., a host processor), for determining the position of the inputobject in the sensing region 120. In one or more embodiments, thereceiver module comprises a plurality of receivers, where each receivermay be an analog front ends (AFEs).

In one or more embodiments, capacitive sensing (or input sensing) anddisplay updating may occur during at least partially overlappingperiods. For example, as a combination electrode is driven for displayupdating, the combination electrode may also be driven for capacitivesensing. Or overlapping capacitive sensing and display updating mayinclude modulating the reference voltage(s) of the display device and/ormodulating at least one display electrode for a display in a time periodthat at least partially overlaps with when the sensor electrodes areconfigured for capacitive sensing. In another embodiment, capacitivesensing and display updating may occur during non-overlapping periods,also referred to as non-display update periods. In various embodiments,the non-display update periods may occur between display line updateperiods for two display lines of a display frame and may be at least aslong in time as the display update period. In such embodiment, thenon-display update period may be referred to as a long horizontalblanking period, long h-blanking period or a distributed blankingperiod. In other embodiments, the non-display update period may comprisehorizontal blanking periods and vertical blanking periods. Processingsystem 110 may be configured to drive sensor electrodes for capacitivesensing during any one or more of or any combination of the differentnon-display update times.

The display driver module 208 includes circuitry confirmed to providedisplay image update information to the display of the display deviceduring non-sensing (e.g., display updating) periods. The display drivermodule 208 may be included with or separate from the sensor module 204.In one embodiment, the processing system comprises a first integratedcircuit comprising the display driver module 208 and at least a portionof the sensor module 204 (i.e., transmitter module and/or receivermodule). In another embodiment, the processing system comprises a firstintegrated circuit comprising the display driver module 208 and a secondintegrated circuit comprising the sensor module 204. In yet anotherembodiment, the processing system comprises a first integrated circuitcomprising a display driver module 208 and one of a transmitter moduleor a receiver module and a second integrated circuit comprising theother one of the transmitter module and receiver module.

The processing system 110 further includes a controller circuit 210 forcontrolling the voltages driven onto the sensor electrodes in the pixels205. As described in more detail below, the controller circuit 210 mayinclude logic circuitry for activating switches (e.g., transistors) toconnect the sensor electrodes to drive lines to drive voltages onto thesensor electrodes and to sense lines for deriving a capacitivemeasurement for the sensor electrodes.

FIG. 3 illustrates a sensor layout 300 for detecting an input object inaccordance with an embodiment of the invention. Although the discussionthat follows describes the sensor layout being used in a fingerprintsensor, the embodiments described herein are not limited to such. Inother embodiments, the components illustrated in FIG. 3 may be used in acapacitive sensing sensor for detecting a position of an input object ina sensing region, whether an input object is hovering over a touchsurface, a palm print, or hand geometry.

As shown, the sensor layout 300 includes multiple sensor electrodes 315which may each form a capacitive sensing pixel 205 as described above.The sensor electrodes 315 are arranged to form a fingerprint sensorarray. As shown, the sensor electrodes 315 are arranged in a matrixpattern that forms a rectangular array. The sensor electrodes 315 areco-planar and disposed on a common substrate. In one embodiment, thewidth and height of the sensor electrodes 315 may range from 5 micronsto 70 microns. Furthermore, the pitch between the sensor electrodes 315is set to enable the sensor layout 300 to detect features in a fingersuch as valleys and ridges. For example, the pitch between sensorelectrodes 315 may range from 5 to 100 microns.

Each sensor electrode 315 is coupled to respective switches 320, 325(e.g., transistors). The switches 320, 325 are controlled (i.e.,activated and deactivated) by sense select lines 305 and drive selectlines 310 which, in one embodiment, are controlled by the controllercircuit 210 shown in FIG. 2. As shown, each row in the sensor layout 300includes a respective pair of the select lines 305, 310. For example,each of the switches 320 and 325 in the upper row are coupled to thesense select line 305A and the drive select line 310A. The switches 320and 325 in the middle row are coupled to the sense select line 305B andthe drive select line 310B, and so forth.

In one embodiment, the sensor electrodes 315 are disposed on a differentsubstrate in the fingerprint sensor than the switches 320, 325 and theselect lines 305, 310. For example, the electrodes 315 may be disposedon a first substrate above a second substrate on which the switches 320,325 are disposed in a display stack. The first substrate may includethrough vias to electrically couple the electrodes 315 to the switches320, 325. In another embodiment, the sensor electrodes 315, switches320, 325, and select lines 305, 310 are disposed on a common substrate.For example, these components may be disposed on the same side of thecommon substrate. However, in another example, the sensor electrodes 315may be disposed on a first side of the common substrate while theswitches 320, 325 and select lines 305, 310 are disposed on a second,opposite side of the common substrate.

In one embodiment, the switches 320, 325 are formed usingthin-film-transistors (TFT). However, in another embodiment, theswitches 320, 325 may be implemented using CMOS transistors. As such thesensor layout 300 may be used in a display stack where switches 320, 325and sensor electrodes 315 are formed using transparent materials (e.g.,transparent TFT) or the sensor layout 300 may be used outside a displaystack where the switches 320, 325 or sensor electrodes 315 are nottransparent (e.g., CMOS technologies or non-transparent TFTs).

The select lines 305, 310 activate the switches 320, 325 which connectthe sensor electrodes 315 to either a sense line 330 or a drive line335. While the switches 320, 325 in the same row of electrodes 315 areall coupled to the same pair of select lines 305, 310, the switches 320in the same column of electrodes 315 are all coupled to the same senseline 330 (also referred to as an output line) while the switches 325 inthe same column of electrodes 315 are all coupled to the same drive line335. When the select line 305A activates the switches 320 in the upperrow, each of the electrodes 315 in the upper row (i.e., electrodes 315A,315B, and 315C) are connected to their respective sense lines 330. Ifonly one of the sense select lines 305 is active at any given time, thenonly one electrode 315 in each column is connected to a sense line 330.For example, if select line 305A is high, but select lines 305B and 305Care low, then only sensor electrodes 315A-C are connected to the senselines 330 while electrodes 315D-I are not.

However, when using CDM to perform capacitive sensing, the input devicemay sense from multiple sensor electrodes 315 in the same columnsimultaneously. As shown, the sense lines 330 are coupled to respectivesensor modules 204. Using CDM, a sensor module 204 can determine thecharge stored on multiple sensor electrodes 315 simultaneously. In oneembodiment, the sensor modules 204 include integrators that integratecharge stored on multiple sensor electrodes connected to the same senseline 330. That is, the sensor modules 204 can sum up the charge storedin the sensor electrodes 315. By repeatedly sensing multiple sensorelectrodes 315 in parallel, the input device can then determine a changeof capacitance for each individual sensor electrode 315. In oneembodiment, the fingerprint sensor uses an N number of combinations ofthe sensor electrodes 315 to determine the individual charge stored onan N number of the sensor electrodes 315. For example, the fingerprintsensor may sense the sum of the sensor electrodes 315 on row 1, row 5,and row 6 on the same column during a first time period and then sensethe sum of the sensor electrodes 315 on row 1, row 2, and row 5 during asecond time period. By using an N number of combinations (where N isequal to or greater than the number of sensor electrodes 315 to besensed), the input device can determine the individual charge stored oneach of the sensor electrodes 315. That is, although with CDM the sensormodules evaluate the charge stored on multiple sensor electrodessimultaneously, backend calculations can be performed to determine theindividual charge (or change in charge) corresponding to each sensorelectrodes 315. Moreover, in one embodiment, the sensor electrodes 315in the same column can be charged to both negative and positivevoltages. For example, the sensor electrode 315 at row 1 (as well as itsneighboring electrodes) could be charged to a negative voltage while thesensor electrodes 315 at row 6 on the same column (and its neighboringelectrodes) could be charged to a positive voltage. By summing thestored charges on the sensor electrodes 315 at row 1 and row 6 asdescribed above, the input device can determine the individual chargesstored on these electrodes 315.

When a drive select line 310 is active, all the sensor electrodes 315 inthe corresponding row are connected to respective drive lines 335. Inone embodiment, the timing of the sense select lines 305 and the driveselect lines 310 is synchronized so that the sense select lines 305 anddrive select lines 310 corresponding to the same row of electrodes 315are not active at the same time. Stated differently, in one embodiment,no electrode 315 is simultaneously connected to both a sense line 330and a drive line 335. The specific timings used to control the switches320, 325 and perform capacitive sensing using the sensor electrodes 315is described in detail below.

In one embodiment, the drive lines 335 are coupled to individual driversthat can drive different voltages. For example, the sensor electrodes315 in the first column can be driven to a HIGH voltage by drive line335A while at the same time sensor electrodes 315 in the second columncan be driven to a LOW voltage by drive line 335B. However, in anotherembodiment, several of the drive lines 335 may be selectively connectedto the same driver. For example, instead of each drive line 335 beingcoupled to an individual driver, several of the drive lines may becoupled to a multiplexer which selectively connects several of the drivelines 335 (up to all of them) to the same voltage source. For example,the multiplexer may connect all the drive lines 335 to a HIGH voltagesource (e.g., a maximum positive voltage), a LOW voltage source (e.g., aminimum negative voltage), or a reference voltage source (V_(REF))(e.g., ground). Thus, when the multiplexer connects the drive lines 335to the HIGH voltage source, the drive lines 335 drive whichever sensorelectrodes 315 are currently connected to the drive lines 335 via theswitches 325 to the HIGH voltage. Coupling the drive lines 335 toindividual drivers may conserve power (because all the drive lines 335are not driven to the same voltage) but connecting the electrodes to thesame voltage source may save space and reduce cost.

FIG. 4 illustrates a sensor module 204A for detecting an input object inaccordance with an embodiment of the invention. The same arrangementshown here may be reproduced for each of the sensor modules 204 in thesensor layout 300. The sensor module 204A includes an operationalamplifier (op amp) 410, feedback capacitor (C_(FB)), and a reset switch405 that is activated using the reset switch signal. If using a CDMtechnique, the sensor module 204A may measure charge on the sensorelectrodes 315A and 315D simultaneously according to the CDM technique.When doing so, the reset switch 405 is deactivated (i.e., open) toprevent the input and output of the op amp 410 from being shortcircuited. In operation, the charge on the sensor electrodes 315A and315D is transferred to the C_(FB) which can be measured and processedusing, for example, a determination module coupled to the output of thesensor module 204A. For example, the sensor module 204A may include ananalog to digital converter which transmits a digital valuecorresponding to the measured charge to the determination module. Usingbackend calculations, the determination module can determine a change incapacitance for each of the sensor electrodes 315. Either the processingsystem 110 in FIG. 1 or a host processor can use the change incapacitance to determine a location of an input object or a fingerprint.

In one embodiment, the amount of charge on the sensor electrodes 315Aand 315B varies depending on the capacitive coupling between the sensorelectrodes 315A and 315B and an input object (e.g., a finger). Forexample, during a first time period, the sensor electrodes 315A and 315Bare charged to a charging voltage. The amount of charge accumulated onthe sensor electrodes 315A and 315B during the first time period dependson the capacitive coupling between the finger and these electrodes.During a second time period, the sensor module 204A measures the amountof charge that was accumulated on the sensor electrodes 315A and 315Bduring the first time period—i.e., the sum of charges on sensorelectrodes 315A and 315B. Using this information, the fingerprint sensorcan determine the capacitive coupling between the electrodes 315A and315B and the finger which can indicate, for example, if a ridge or avalley of the finger is disposed over the electrodes 315A and 315B.

However, the capacitance between the electrodes 315A and 315B and theinput object (finger) is not the only capacitance in the fingerprintsensor. There is also capacitive coupling between the sensor electrodes315A and 315B and neighboring electrodes (not shown in FIG. 4). In fact,this capacitive coupling may be orders of magnitude larger than thedifference of the capacitive coupling between a ridge in the finger tothe electrodes 315A and 315B and the capacitive coupling between avalley in the finger to the electrodes 315A and 315B. Moreover,capacitive coupling from the sense line 330A and the drive lines 335(and other neighboring sense and drive lines) to the sensor electrodes315A and 315B may also affect how much charge is accumulated on thesensor electrodes 315A and 315B during the first time period. Thesecapacitive couplings are collectively referred to herein as parasiticcapacitances which make detecting the capacitive coupling between theinput object and the sensor electrodes 315A and 315B more difficult tomeasure.

As used herein, a neighboring sensor electrode is a sensor electrodewhose capacitive coupling to the electrode to be sensed interferes withthe ability of a sensor module to measure a desired capacitance betweenthe selected electrode and the input object. For example, when used as afingerprint sensor, a neighboring sensor electrode may be a sensorelectrode whose capacitive coupling to the selected electrode is atleast on the same order of magnitude as the difference of the capacitivecoupling between a ridge in the finger to the selected electrode and thecapacitive coupling between a valley in the finger to the selectedelectrode. Moreover, an adjacent sensor electrode is an electrode thatis directly next to the sensed electrode in the matrix, either at anangle or along a row or column. For example, referring to FIG. 3, thesensor electrode 315C can be a neighboring electrode to sensor electrode315A even though electrode 315C is not directly adjacent to sensorelectrode 315A.

Embodiments described herein use common mode cancellation to remove ormitigate the effects of the parasitic capacitance between the sensorelectrodes 315A and 315B and their neighbors—i.e., the sensor electrodesto which the electrodes 315A and 315B are capacitively coupled. To doso, the fingerprint sensor ensures there is no voltage differencebetween the sensor electrodes 315A and 315B and their neighboringelectrodes. If the voltage difference is zero, the charge accumulated onthe sensor electrodes 315A and 315B is affected only by the capacitancecoupling to the input object.

FIG. 5 is a flowchart of a method 500 for controlling neighboringelectrodes when performing capacitive sensing in accordance with anembodiment of the invention. At block 505, the controller in theprocessing system charges at least two electrodes connected to the samesense line and at least one neighboring electrode to a first voltageduring a charge stage. For clarity, the method 500 is discussed intandem with FIGS. 6A-6C which illustrate sensing patterns for operatingsensor electrodes in accordance with an embodiment of the invention.FIGS. 6A-6C illustrates a matrix sensor 600 that includes sensorelectrodes (illustrated as individual squares) arranged in six rows andfive columns. Although not shown, it is assumed the sensor electrodesare connected to sense and drive lines and select lines as shown in FIG.3. For example, all the sensor electrodes in the same row are coupled tothe same sense and drive select lines while all the sensor electrodes inthe same column are selectively coupled to the same sense and drivelines.

FIGS. 6A-6C illustrate one example of performing the method 500 in orderto measure the charge (or change in charge) on the two electrodeslabeled sensed electrodes 605. Specifically, FIG. 6A illustratesperforming the charge stage as described in block 505 of method 500. Asshown, the sensed electrodes 605 along with neighboring electrodes 610that are directly adjacent to the sensed electrodes 605 are driven tothe charging voltage (e.g., HIGH voltage). That is, in this embodiment,the neighboring electrodes 610 include the electrodes that surround thesensed electrodes 605.

The hashing in the sensed electrodes 605 and the neighboring electrodes610 illustrates that these electrodes are driven to a high voltageduring the charge stage. That is, these sensor electrodes are connectedto the drive lines 335 in FIG. 3 which connect the electrodes to one ormore high voltage sources. Because the voltage difference between theneighboring electrodes 610 and the sensed electrodes 605 during thecharge stage remains zero, the parasitic capacitive coupling betweenthese electrodes does not affect the amount of charge stored in thesensed electrodes 605. However, if an input object is capacitivelycoupled to the sensed electrodes 605, then this capacitance does affectthe amount of charge stored in the sensed electrodes 605 during thecharge stage, and thus, affect the amount of charge detected during theread stage discussed below.

While the sensed electrodes 605 and the neighboring electrodes 610 aredriven to the high voltage, the surrounding electrodes in row 1 and 6and columns 1 and 5 are electrically floating. In one embodiment, thecorresponding switches 320 and 325 for each of these sensor electrodesare deactivated (i.e., open) such that the sensor electrodes are notconnected to either the sensor lines or the drive lines. However, inother embodiments, the electrodes in row 1 and 6 and columns 1 and 5 canalso be driven to the high voltage during the charge stage whichprevents any capacitive coupling between these electrodes and the sensedelectrodes 605 from affecting the amount of charge stored on the sensedelectrode 605 during this stage. However, driving these electrodesrequires power. Thus, the system designer may balance between mitigatingthe parasitic capacitance between the sensed electrodes 605 and theneighboring electrodes and the power consumption of the matrix sensor600. In FIG. 6A, only the neighboring electrodes 610 that are directlyadjacent to the sensed electrodes 605 are driven to high voltage, but inother examples all the sensor electrodes can be driven to the highvoltage.

Although FIG. 6A illustrates driving the sensed electrodes 605 and theneighboring electrodes 610 to a high voltage, in other cycles of a CDMtechnique (where method 500 is repeated), these electrodes may be drivento a negative voltage. In one example, the high and low voltages may be+/−20V relative to a reference voltage (e.g., virtual ground).

Returning to method 500, at block 510, the controller discharges atleast one of the neighboring electrodes to a reference voltage during aprecondition stage. Moreover, the sensed electrodes are electricallyfloating during the precondition stage. In one embodiment, theprecondition stage is used to ensure that the sensed electrodes aredisconnected from the drive lines (which drives a high or low voltageonto the sensed electrodes) before the sensed electrodes are connectedto the sense lines so that the AFE in the sensor module can measure thecharge on the sensed electrodes. Put differently, the precondition stagecan provide a length of time to sequentially activate the switches 320and 325 in FIG. 3 coupled to the sensed electrodes so that theseelectrodes are not connected to the sense and drive linessimultaneously.

FIG. 6B illustrates driving the matrix sensor 600 during theprecondition stage. As shown, the neighboring electrodes 605 in rows 2and 5 are driven to the reference voltage V_(REF) while the neighboringelectrodes 610 in rows 3 and 4 along with the sensed electrodes 605 areelectrically floating. In one embodiment, the switches coupling thesensor electrodes in rows 3 and 4 to the drive lines are deactivatedsuch that these rows are no longer connected to the drive lines. Oncerows 3 and 4 are disconnected from the drives lines, the processingsystem can change the voltage on the drives lines corresponding tocolumns 2-4 to the reference voltage which causes the voltage on theneighboring electrodes in rows 2 and 5 to decrease to V_(REF). However,because rows 3 and 4 are disconnected from the drive lines (i.e., areelectrically floating), the charge stored in the neighboring electrodes610 and the sensed electrodes in rows 3 and 4 does not change.

In another embodiment, instead of driving the voltage on the neighboringelectrodes 610 in rows 2 and 5 to V_(REF) using the drive lines, theprocessing system may disconnect these electrodes from the drive linesand instead connect these electrodes to the sense lines which may beconstantly driven to V_(REF). By making this switch, the neighboringelectrodes 610 in rows 2 and 5 are driven to V_(REF) without having tochange the voltage on the drive lines for columns 2-4.

In a further embodiment, it is not necessary that the voltage on theneighboring electrodes 610 in rows 2 and 5 be driven to V_(REF) duringthe precondition stage. Instead, these electrodes can be driven toV_(REF) during the subsequent read stage. For example, during theprecondition stage the neighboring electrodes 610 in rows 2 and 5 can beelectrically floated by disconnecting these electrodes from the drivelines in columns 2-4 (like what is done with the sensor electrodes inrows 3 and 4).

Returning to the method 500, at block 515, the controller discharges theremaining electrodes to the reference voltage during the read stage. Inone embodiment, the processing system drives any of the neighboringelectrodes that were driven to the high voltage during the chargingstage to the reference voltage if the electrodes were not previouslydriven to the reference voltage. FIG. 6C illustrates driving the matrixsensor 600 during the read stage. As shown, the neighboring electrodes610 in rows 3 and 4 that were not driven to V_(REF) during theprecondition stage shown in FIG. 6B are connected to the sense lineswhich drive their voltages to V_(REF).

Returning to method 500, at block 520, the controller discharges the atleast two electrodes (i.e., the sensed electrodes) to the referencevoltage while integrating the sum of their charges during a read stage.That is, the sensed electrodes, which are electrically floating duringthe precondition stage, are connected to the sense lines during the readstage. As shown in FIG. 4, the positive terminal of the op amp 410 iscoupled to V_(REF) which drives the sense line 330A (and the sensorelectrodes 315A and 315B when the switches 320 are active) to V_(REF).Thus, when the sensor electrodes 315A and 315B are connected to thesense line 330A, the sensor module 204A begins to integrate the chargestored in these sensor electrodes 315A and 315B during the charge stage.

FIG. 6C illustrates driving the matrix sensor 600 during the read stage.In this embodiment, the sense select lines for rows 3 and 4 areactivated thereby connecting the electrodes in these rows to theirrespective sense lines. Assuming the sense lines are driven to V_(REF)by the sensor module, then during the read stage the sensor electrodesin rows 3 and 4 are also driven to V_(REF).

The neighboring electrodes in rows 2 and 5 may remain connected to thedrive lines during the read stage which maintains these electrodes atthe reference voltage. Alternatively, the neighboring electrodes in rows2 and 5 can also be connected to the sense lines during the read stagebut it may be preferred to maintain connection to drive lines on column3 (the sense electrode column) since disconnecting the rows from thedrive line and connecting to sense lines can produce residual chargetransfer or noise which may harm capacitive sensing. Further, theelectrodes at row 3, columns 1 and 5 as well as the electrodes at row 4,columns 1 and 5 are at V_(REF) if the corresponding sense lines areconnected to V_(REF). However, these electrodes would be floating iftheir corresponding sense lines are floating. Because these sensorelectrodes are driven to V_(REF) during the precondition stage shown inFIG. 6B which depletes their charge, coupling the sensor electrodes inrows 2 and 5 to the same sense line as the sensed electrodes 605 doesnot affect the charge measured by the sensor module during the readstage. Stated differently, coupling the neighboring electrodes directlyabove and below the sensed electrodes 605 to the sense line shared withthe sensed electrodes 605 does not affect the charge measured by thesensor module so long as the neighboring electrodes were driven to thereference voltage previously.

In one embodiment, blocks 515 and 520 are performed in parallel. Inanother embodiment, the functions described in blocks 515 and 520 may beperformed, at least partially, during non-overlapping time periods butend at the same time.

During the method 500, the neighboring electrodes 610 and the sensedelectrodes 605 are driven to a high voltage (which can be a negative ora positive voltage) and then driven to V_(REF). Because the voltagedifference between the neighboring electrodes 610 and the sensedelectrodes 605 remains constant (zero in this example), the effect ofthe parasitic capacitance between the neighboring electrodes 610 and thesensed electrodes 605 when charging the sensed electrodes during thecharge stage and when depleting the charge during the read stage isreduced. Thus, the capacitive sensing measurements acquired during theread stage using the sensed electrodes 605 may more accurately representthe capacitance between the sensed electrodes 605 and the input object,e.g., a user's finger.

The processing system can repeat the method 500 on the matrix sensor 600to perform CDM. Although FIGS. 6A-6C illustrate measure capacitance ontwo of the electrode simultaneously, in other embodiments, theprocessing system may use four, eight, or more sensed electrodes whichare measured simultaneously during the read stage. Furthermore, theprocessing system may perform the method 500 on sensed electrodes indifferent columns. For example, while performing the charge,precondition, and read stage on the sensed electrodes 605 shown in thematrix sensor 600, the processing system may also perform the sameactions on sensed electrodes and neighboring electrodes in otherlocations in the matrix sensor 600. For example, the processing systemmay charge, precondition, and read sensor electrodes in column 10 at thesame time the sense electrodes 605 in column 3 are charged,preconditioned, and read. Further, the processing system may drivedifferent polarities onto the different sensed electrodes in thecolumns. For example, during the charge stage, the processing system maydrive the sensed electrodes 605 in column 3 to a positive high voltagewhile driving the sensed electrodes in column 10 to a negative highvoltage. In this manner, the processing system can charge, precondition,and read on multiple electrodes in each column in the matrix sensor 600in parallel. However, this assumes the processing system has arespective sensor module coupled to each sense line. If multiple senselines are multiplexed to the same sensor module—i.e., there are fewersensor modules than sense lines—the processing system measures chargefrom only the sensor electrodes that are currently connected to thesensor modules.

In one embodiment, it may be desired to drive the sensed electrodes 605to different polarity voltages during multiple iterations of method 500if the AFEs are differential. For example, method 500 may be repeatedfor the sensed electrodes 605 except that instead of driving the sensedelectrodes 605 and neighboring electrodes 610 to a positive voltageduring the charge stage, these electrodes are driven to a negativevoltage.

One advantage of operating the sensor layout 300 in FIG. 3 using themethod 500 shown in FIG. 5 is that the drive and sense lines areseparate which means the high voltages used during the charging stageare separated from the readout function performed by the sense lines andthe sensor modules. For example, the voltages on the drive lines mayrange from +/−50 V while the voltages on the sense line may never exceed+/−1 V. As a result, the AFE in the sensor module may operate in avoltage range less than +/−1 V which means the design of the AFE can besimpler, more cost effective, and conserve power.

FIG. 7 is a timing diagram 700 corresponding to the sensor layout shownin FIG. 6A in accordance with an embodiment of the invention. The x-axisof the diagram 700 represents time and is subdivided into the charge,precondition, and read stages. The y-axis lists the states of the driveselect lines, drive lines, and sense select lines for various rows andcolumns in the matrix sensor 600 in FIG. 6A. During the charge stage,the drive select lines for rows 3 and 4 are ON which connects the sensorelectrodes in rows 3 and 4 (which includes the sensed electrodes) to thedrives lines. The drive select lines for rows 2 and 5 are also ON duringthe charge stage since these rows include electrodes that are directlyadjacent to the sensed electrodes in rows 3 and 4.

The drive lines for columns 2-4 which includes the directly adjacentneighboring electrodes and the sensed electrode are driven to the HIGHvoltage during the charge stage. The sense select lines for rows 3 and 4are maintained in the OFF state so that the electrodes in these rows arenot simultaneously connected to the drive lines and the sense lines.Upon the completion of the charge stage, the voltages on the sensedelectrodes and the neighboring electrodes are in the state shown by FIG.6A.

During the precondition stage, the drive select line for rows 3 and 4are turned OFF thereby disconnecting the sensor electrodes in these rowsfrom the drive lines (which are still at the HIGH voltage at thebeginning of the precondition stage). Because the sense select lines forrows 3 and 4 are also OFF during the precondition stage, the electrodesin these rows are electrically floating like as in shown in FIG. 6B.Approximately half way through the precondition stage, the drive linesfor columns 2-4 switch from driving the HIGH voltage to driving thereference voltage V_(REF). Because the drive select lines for rows 2 and5 are still ON, the electrodes in rows 2 and 5 and columns 2-4 aredriven to V_(REF) as shown in FIG. 6B. Although shown as switching halfway through the precondition stage, the drive lines can switch toV_(REF) as soon as the switches (e.g., TFTs) to the sensing electrodesare completely turned off.

During the read stage, the sense select lines for rows 3 and 4 are ONwhich connects each electrode in these rows to a sense line. Doing sodrives the sensor electrodes in rows 3 and 4 to V_(REF). In parallel, anAFE in the sensor module coupled to the sensed electrodes can measurethe charge stored in the sensed electrode during the charge stage, whichcan then be processed to determine a capacitance measurement for each ofthe individual sensed electrodes.

FIGS. 8A and 8B illustrates a sensor layout 800 for detecting an inputobject in accordance with an embodiment of the invention. The sensorlayout 800 includes a sensor electrode 315 that is selectively connectedto a sense line 820 via a switch 830 and to a drive line 825 by either aswitch 835 or a switch 840. That is, unlike in FIG. 3 where the sensorelectrodes 315 are connected to the drive line 335 by a single switch325, in the sensor layout 800 each sensor electrode 315 is connected tothe drive line 825 by two switches. The switch 835 is controlled by adrive row select line 810 (which is arranged in a similar position asthe drive select line 310 in FIG. 3), while switch 840 is controlled bya drive column select line 815 that extends parallel to the drive line825.

Adding the switch 840 and the drive column select line 815 permits theprocessing system to connect a neighboring electrode in the same row asa sensed electrode to the drive line when the sensed electrode isfloated. Using FIG. 6B as an example, during the precondition stage, theneighboring electrodes 610 in the same rows as the sensed electrodes 605(i.e., rows 3 and 4) are electrically floated since both the senseselect line 305 and the drive select line 310 in FIG. 3 are OFF.However, if the sensor layout 800 in FIG. 8A was instead used, duringthe precondition stage the neighboring electrodes 610 in the same rowsas the sensed electrodes 605 can be driven by the drive lines to V_(REF)the same as the neighboring electrode 610 in rows 2 and 5. That is,during the precondition stage, the processing system activates the drivecolumn select line 815 which turns ON the switch 840 and connects thesensor electrode 315 to the drive line 825 which drives the sensorelectrode 315 to V_(REF). Thus, if sensor layout 800 was used to formthe matrix sensor 600 shown in FIG. 6B, the neighboring electrodes 610in rows 3 and 4 (i.e., the electrodes to the left and right of thesensed electrodes 605) are driven to V_(REF) like the other neighboringelectrodes 610 rather than being electrically floating as shown.

FIG. 8B illustrates a sensor layout 850 similar to the sensor layout 800except that the two switches 835 and 840 are replaced by a dual gateswitch 855 which is activated either by drive row select line 810 or bydrive column select line 815. That is, the drive row select line 810 andthe drive column select line 815 can independently activate the dualgate switch 855 and connect the sensor electrode 315 to the drive line825. Thus, the sensor layout 850 can perform the function recited abovewhere if the sensor electrode 315 is disposed on the same row as asensed electrode, the electrode 315 can be driven to V_(REF) during theprecondition stage.

Noise Mitigation when Performing CDM

As described above, when performing CDM, multiple sensor electrodes arecoupled to the same sensor module (e.g., the same AFE) and are chargedto different voltages according to a predefined code. As such, thesensor module generates a measurement that represents the sum of thecharges (e.g., a combined charge) on the sensor electrodes rather thanan individual charge on a single sensor electrode. By repeatedly sensingmultiple sensor electrodes in parallel with different sets of voltagesaccording to the code, the input device can determine a change ofcapacitance for each individual sensor electrode. In one embodiment, thecode used to perform CDM includes an N number of invertible combinationsof the sensor electrodes (which can be expressed in a two-dimensionalmatrix) to determine the individual charge stored on an N number of thesensor electrodes. By using an N number of combinations (where N isequal to or greater than the number of sensor electrodes to be sensed),the input device can determine the individual charge stored on each ofthe sensor electrodes. Backend calculations can be performed todetermine the individual charge (or change in charge) corresponding toeach sensor electrode. Although CDM is specifically described, theembodiments herein can be used with any sensing technique that usespredefined codes to drive the sensor electrodes. In one embodiment, thecodes can be any, invertible sequence of voltages that are driven on thesensor electrodes.

When performing CDM, the transistors that connect the sensor electrodesto the sensing and drive lines (and to the sensor module) can injectcharge. For example, field effect transistors (e.g., a PFET or NFET)have a capacitance between the gate and the source and drain. Thus,driving a signal on the gate can inject charge (e.g., noise) onto boththe source and drain which may be coupled to the sensor electrodes. Inanother example, when a transistor is deactivated (e.g., the gate isturned off), the charge in the conductive channel is forced out of theintrinsic region into the source and drain connections which can alsoinject charge and noise into the sensor module.

To mitigate charge injection from switching the transistors, in oneembodiment, each column in the sensor array includes an offset sensorelectrode. For example, instead of coupling all of the sensor electrodesin the same column to the sensor module when capturing a capacitivesensing measurement, at least one of the sensor electrodes in the columnis selected as an offset sensor electrode. The offset electrodes are notsensed but rather their sense select switches are driven to compensatefor the charge injection described above. During a subsequent CDM cycleor measurement, the sensor electrodes designated as the offset sensorelectrodes are switched such that each sensor electrode in the column issensed at least once. For example, each column of a sensor electrodesmay be sensed eight times where the sensor electrodes designated asoffset sensor electrodes changes.

In addition, the input device may perform double correlated samplingwhich can mitigate the noise injected by the reset switch (ortransistor) in the sensor module. In one embodiment, the reset switch isswitched when the offset sensor electrodes are coupled to the sensormodule. The senor module then samples the total charge on the offsetsensor electrodes. The sense select lines are then controlled so thatthe offset sensor electrodes are disconnected from the sensor modulewhile the remaining sensor electrodes in the column are connected to thesensor module and sampled. The input device can use double correlatedsampling to identify and remove the charge injected from the resetswitch from the capacitive sensing measurement.

FIG. 9 illustrates a timing chart for using offset sensor electrodeswhen performing capacitive sensing in accordance with an embodiment ofthe invention. For clarity, FIG. 9 is discussed using the sensor layout300 shown in FIG. 3. That is, each sensor electrode 315 is selectivelycoupled to either a sense line 330 or a drive line 335 to performcapacitive sensing. When performing CDM, multiple sensor electrodes 315in a column are connected to the same sensor module 204 which measures atotal charge on the sensor electrodes 315. Moreover, the functionsdescribed in FIG. 9 may be performed by the controller circuit 210 inthe processing system 110. For example, the controller circuit 210 mayinclude logic for controlling the gate lines, driving voltages onto thesensor electrodes, controlling the select lines, controlling the resetswitch in the sensor module, etc.

In one embodiment, at least one of the sensor electrodes 315 in thecolumn is designated as an offset sensor electrode while the remainingsensor electrodes 315 in the column are designated as selected sensorelectrodes. The offset sensor electrodes are used to compensate for thecharge injection generated when switching the transistors in the layout300—e.g., switches 320, 325, and the reset switch in the sensor modules204. In another embodiment, the input device may include dedicatedoffset switches that are located outside of the layout 300. For example,the dedicated offset switches may compensate for the charge injectiongenerated by the switches 320 and 325 in the sensor layout 300 but arenot coupled to the sensor electrodes 315 that are used for capacitivesensing.

FIGS. 10A-10E illustrate operational states of a column 1000 of sensorelectrodes during various time periods illustrated in FIG. 9 inaccordance with embodiments of the invention. Specifically, FIGS.10A-10E correspond to one column 1000 of sensor electrodes 315 shown inthe sensor layout 300 which illustrates the electrical connections ofthe offset and selected sensor electrodes during the time periods inFIG. 9. FIG. 10A illustrates a column 1000 of sensor electrodes 315 inthe layout 300 where two of the sensor electrodes are designated asoffset sensor electrodes and two of the sensor electrodes are designatedas selected sensor electrodes during the CDM cycle illustrated in FIG.9. Each of the sensor electrodes are connected to the switch 320 whichselectively couples the sensor electrode to the sense line 330 and thesensor electrodes are connected to the switch 325 which selectivelycouples the sensor electrode to the drive line 335. For clarity, thesense select lines 305 and the drive select lines 310 are not shown.

At Time A in FIG. 9, the selected sensor electrodes are disconnectedfrom both the drive line 335 and from the sense lines 330, and as aresult, are electrically floating. The offset sensor electrodes in eachcolumn of the layout 300, however, are connected to their respectivesense lines 330, and thus, are connected to one of the sensor modules204. Although not shown in FIG. 9, the offset sensor electrodes remaindisconnected from the drive line 335 through Times A-G.

Moreover, as shown by the RESET signal in FIG. 9, the reset switch inthe sensor module 204 is closed which drives the sense line 330 to areference voltage V_(REF) (e.g., ground). Because the offset sensorelectrodes are coupled to the sense line 330, these electrodes are alsodriven to the reference voltage V_(REF).

FIG. 10B illustrates the operational state of the column 1000 duringTime A in FIG. 9. For the selected sensor electrodes 1010, the switches320 and 325 are open which means these electrodes 1010 are disconnectedfrom both the drive line 335 and the sense lines 330, and thus, areelectrically floating. In contrast, the offset sensor electrodes 1010are connected to the sense line 330 by the switches, and thus, aredriven to the reference voltage V_(REF) by the sensor module 204.

At Time B in FIG. 9, the input device controls the drive select lines310 to couple the selected sensor electrodes to the drive lines 335.FIG. 9 illustrates a +1 selected electrode drive select signals and a −1selected electrode drive select signal. A CDM code may include multiplecycles or iterations where during each cycle one or more of the selectedsensor electrodes are driven to a positive voltage (+1) while one ormore of the selected sensor electrodes are driven to a negative voltage(−1). During subsequent cycles or iterations, the voltages driven on theselected sense electrodes can switch where a sensor electrode that wasdriven to a negative voltage is now driven to a positive voltage and asensor electrode that was driven to a positive voltage is now driven toa negative voltage. However, the voltages for the +1 and −1 electrodesdo not need to be positive and negative relative to the referencevoltage V_(REF). For example, the electrodes corresponding to the +1selected electrode drive select signals can a first positive voltagewhile the electrodes corresponding −1 selected electrode drive selectsignal are driven to a second, different positive voltage relative toV_(REF). Alternatively, the +1 and −1 sensor electrodes could be drivento two different negative voltages relative to the reference voltageV_(REF). That being said, using positive and negative voltages relativeto V_(REF) (which have equal magnitudes) can improve the signal measuredby the sensor module 204.

The signal V_(tx) represents the voltage driven on the drive line 335.In this embodiment, at Time B, the selected sensor electrodes are drivento −7.5V. However, this is exemplary and other voltages can be used.

FIG. 10C illustrates the operational state of the column 1000 duringTime B in FIG. 9. As shown, the selected sensor electrodes 1010 arecoupled to the drive line 335 while the offset sensor electrodes 1005are coupled to the sense line 330. Thus, the selected sensor electrodes1010 are driven to V_(tx) (i.e., −7.5V as shown in FIG. 9).

At Time C in FIG. 9, the “−1” selected sensor electrodes aredisconnected from the drive lines 335 while the “+1” selected sensorelectrodes remain coupled to the drive lines 335. Moreover, the voltageV_(tx) (driven on the drive lines 335 changes from a negative voltage toa positive voltage—e.g., +7.5 V. That is, the selected sensor electrodesthat are designated to have positive voltages according to the CDM coderemain coupled to the drive lines 335 so that their voltages are drivento +7.5V. In contrast, the selected sensor electrodes that should havenegative voltages during the CDM cycle are disconnected from the driveslines 335, and thus, remain at −7.5 V.

While FIG. 9 illustrates driving all the selected sensor electrodes 1010to the negative voltage before driving the “+1” selected sensorelectrodes 1010 to the positive voltage, in one embodiment, the inputdevice may drive, at Time B, only the “−1” selected sensor electrodes1010 to the negative voltage, and at Time C, drive only the “+1”selected sensor electrodes 1010 to the positive voltage. However,driving all the selected sensor electrodes the same voltage at Time C asshown in FIG. 9 may save time.

Referring again to FIG. 10C, if one of the selected sensor electrodes1010 is a “−1” sensor electrode and the other one electrode 1010 is a“+1 electrode, then at Time C, only one of the sensor electrodes 1010 iscoupled to the drive line 335 while the other is disconnected from thedrive line 335. Thus, one of the selected sensor electrodes 1010 has apositive voltage while the other selected sensor electrode 1010 has anegative voltage.

At Time D in FIG. 9, the reset switch in the sensor module 204 isopened. However, the transistor forming the reset switch may injectcharge onto the sense line 330 which can affect the measurementgenerated by the sensor module 204. To compensate for this chargeinjection, the sensor module 204 performs a double correlated sampling:at least one sample is captured at Time E, while another sample iscaptured at Time G as shown by the demodulator signal. Any chargemeasured by the sensor module 204 at Time E is dependent on the chargeinjected when opening or deactivating the reset switch. In this manner,the input device can determine the charge injected by the reset switch.

FIG. 10D illustrates the operational state of the column 1000 duringTime E in FIG. 9. As shown, the selected sensor electrodes 1010 areelectrically floating (but have been previously charged to the positiveand negative voltages during Times B and C). The offset sensorelectrodes 1005 remain coupled to the sense line 330, and thus, there isno net charge on the electrodes 1005 which is sampled during Time E bythe sensor module 204. Because the offset sensor electrodes 1005 werepreviously driven to the reference voltage, the measurement obtained bythe sensor module 204 may depend primarily on the charge injected fromswitching the reset switch 405 to the open state as shown in FIG. 10D.The negative effects associated with the charge injected by the resetswitch 405 can be mitigated by the backend processing performed in theinput device. In one embodiment, the input device uses a matched filterfor removing the charge injected from the reset switch 405 from thecapacitive sensing measurements.

At Time F, the input device uses the sense select lines 305 to couplethe selected sensor electrodes to the sense line 330. In parallel, theinput device disconnects the offset sensor electrodes from the senseline 330. Thus, for each column in the array, only the selected sensorelectrodes are coupled to the sense line 330 while the offset sensorelectrodes are not.

Figure WE illustrate the operational state of the column 1000 duringTime F in FIG. 9. As shown, the switches 320 corresponding to the offsetsensor electrodes (also referred to herein as “offset switches” or“offset transistors”) are opened while the switches 320 corresponding tothe selected sensor electrodes 1010 are closed, thereby coupling theselected sensor electrodes 1010 to the sense line 330. As mentionedabove, changing the state of the switches 320 can inject charge which ismeasured by the sensor module 204. The offset sensor electrode switchesare used to compensate for the charge injection. In one embodiment, theamount of charge injected (or sunk) by activating the switches 320corresponding to the selected sensor electrodes 1010 is such (orinjected) by deactivating the offset switches 320 corresponding to theoffset sensor electrodes 1005. Thus, the overall charge measured by thesensor module 204 due to toggling the switches 320 (e.g., thetransistors) can be reduced to near zero.

In one embodiment, the amount of charge injected by a transistor may bedifferent depending if the transistor is being activated (e.g., closed)or deactivated (e.g., opened). As such, the number of the sensorelectrode in a column that are designated as offset sensor electrodesand selected sensor electrodes may be different. That is, the column mayneed to have a different number of offset sensor electrodes 1005 tosufficiently compensate for the charge injected by closing the switches320 corresponding to the selected sensor electrodes 1010. However, inone embodiment, the column may have the same number of the offset sensorelectrodes 1005 as the selected sensor electrodes 1010. In that case,the charge injection may be not completely balanced by the switches 320but may be sufficiently balanced such that the injected charge does notprevent the sensor module 204 from accurately measuring the charge onthe selected sensor electrodes 1010.

As mentioned above, instead of assigning electrodes in the column 1000as offset sensor electrodes, in one embodiment, the columns 1000 in thearray can include dedicated offset switches or offset transistors whichare activated at Time F to compensate for the charge injected by theswitches 320 coupled to the sensor electrodes. In one embodiment, thededicated offset switches are used only for compensating for the chargeinjection from activating and/or deactivating the switches 320. Stateddifferently, the offset switches may not be coupled to sensorelectrodes. As such, the offset switches can be disposed external to thelayout 300 and the fingerprint sensor. In one embodiment, the offsetswitches are disposed in the processing system.

The dedicated offset switches are electrically coupled to the sense line330. Thus, when the switches 320 are activated at Time F to connect thesensor electrodes to the sense line 330, the dedicated offset switchescan be deactivated in order to compensate for the charge injected by theswitches 320. As such, an input device that includes dedicated offsetswitches does not need the offset sensor electrodes 1005. That is,instead of having offset sensor electrodes where the charge is notmeasured during Time F, all of the sensor electrodes in the column 1000can measured during the CDM cycles. In that case, each of the sensorelectrodes in the column 1000 can be the selected sensor electrodes1010. However, in one embodiment, the dedicated offset switches may becoupled to dummy offset sensor electrodes (i.e., sensor electrodes thatare not used to perform capacitive sensing) since the load on thededicated offset switches (e.g., the dummy offset sensor electrodes)effects the charge injected when activating or deactivating the offsetswitches. The dummy offset sensor electrodes do not need to be in anactive area of the fingerprint sensor but can be disposed elsewhere.

At Time G, the sensor module 204 samples the charge in the selectedsensor electrodes 1010. Because the offset switches 320 coupled to theoffset sensor electrodes 1005 compensate for the charge injected byactivating the switches 320 coupled to the selected sensor electrodes1010, the remaining charge is mainly from the sensor electrodes 1010 andnot from the charge injection of switches 320.

Although not shown in FIG. 9, when the demodulator signal changes atTimes E and G, the sensor module 204 may capture multiple samples. Forexample, at Time E, the sensor module 204 may sample the charge on theoffset sensor electrodes 1005 multiple times. Similarly, at Time G, thesensor module 204 can sample the total charge on the selected sensorelectrodes 1010 any number of times. Furthermore, a continuous timesampling scheme may be used where the demodulator corresponds to acontinuous time mixer.

In one embodiment, FIG. 9 is modified such that the voltage V_(tx) onthe drive line 335 only transitions between negative and positivevoltages. That is, the voltage V_(tx) does not stay at the referencevoltage (e.g., ground). In this embodiment, the voltage V_(tx) remainsat +7.5V during Times D-G instead of changing to the reference voltageas shown. By keeping the voltage at the positive voltage during theremaining portion of the CDM cycle, the amount of charge injected viaparasitic capacitances from drive line 335 to sense line 3430 by thepositive voltage transition cancels the amount of charge injected by thenegative voltage transition. That is, the effect of the transition inV_(tx) from the negative to the positive voltage on the sensor module204 cancels if the voltage remains at the positive voltage. As such, theop amp in the sensor module 204 does not need to be held at zero (e.g.,the reset switch 405 does not need to be closed). As a result, thesensor module 204 may not need to use the reset switch 405 every cycleand may only need to use it once per burst. In one embodiment, the resetswitch 405 is replaced by a DC feedback element such as a resistor toprevent the sensor module 204 from drifting.

FIG. 11 illustrates a timing chart for using offset sensor electrodeswhen performing capacitive sensing in accordance with an embodiment ofthe invention. The timing chart illustrates the signals for driving oneof the columns in the sensor layout 300 in FIG. 3 where a plurality ofsensor electrodes 315 can be selectively coupled to the same sensormodule 204. Each of the columns in the layout 300 can be sensed inparallel using timing signals illustrated in FIG. 11. Moreover, thecolumn includes at least one offset sensor electrode and at least oneselected sensor electrode as discussed above. Further, like in FIG. 9,the functions described in FIG. 11 may be performed by the controllercircuit 210 in the processing system 110. For example, the controllercircuit 210 may include logic for controlling the gate lines, drivingvoltages onto the sensor electrodes, controlling the select lines,controlling the reset switch in the sensor module, etc.

At Time A in FIG. 11, the selected sensor electrodes are coupled to thedrive line 335 such that the voltage V_(tx) of −7.5V is driven ontothese electrodes. That is, both the “−1” selected sensor electrodes andthe “+1” selected sensor electrodes are driven to the negative voltage−7.5V. Moreover, the selected sensor electrodes are not coupled to thesense line 330. The offset sensor electrodes, in contrast, are notcoupled to the drive line 335 but are coupled to the sense line 330 andto the sensor module 204.

At Time B in FIG. 11, the “−1” selected sensor electrodes are notcoupled to the drive line 335 while the “+1” selected sensor electrodesremain coupled to the drive line 335. Thus, the voltage on the “+1”selected sensor electrodes is driven to the positive voltage 7.5V by thevoltage V_(tx).

At Time C, the selected sensor electrodes are no longer coupled to thedrive line 335. Further, the selected sensor electrodes are not coupledto the sense line 330, and thus, are electrically floating. However, the“−1” selected sensor electrodes remain at the negative voltage −7.5Vwhile the “+1” selected sensor electrodes are at the positive voltage7.5V.

At Time D in FIG. 11, the selected sensor electrodes are coupled to thesense line 330 while the offset sensor electrodes are no longer coupledto the sense line 330. As shown by the demodulator signal, the inputdevice uses the sensor module 204 to obtain one or more samplesrepresenting the charge on the selected sensor electrodes in the column.Further, the positive change in the demodulator signal indicates thatthe input device performs a positive demodulation on the resultingdigital data obtained from the samples.

At Time E in FIG. 11, the selected sensor electrodes are disconnectedfrom the sense line 330 and coupled to drive line 335, and as a result,are driven to the negative voltage −7.5V. At Time F, the “−1” selectedsensor electrodes are driven to the positive voltage 7.5V while the “+1”selected sensor electrodes remain charged to the negative voltage −7.5V.Thus, the polarities of the voltages on the selected sensor electrodesare reversed relative to the voltages on the selected sensor electrodesat Time C.

At Time G, the selected sense electrodes are again coupled to the senseline 330 and the sum of the charge on the selected sense electrodes issampled by the sensor module 204. The negative change in the demodulatorsignal indicates that the input device performs a negative demodulationon the resulting digital data obtained from the captured samples insteadof performing a negative demodulation like at Time D.

FIG. 11 illustrates a CDM cycle that has an even portion (e.g., TimesA-D) and an odd portion (e.g., times E-G). The even and odd portions canbe considered to each be a positive and a negative half of a single CDMcycle.

During the even portion, the selected sensor electrodes are driven torespective voltages at Times A and B and then a positive demodulation isperformed at Time D. During Times E and F in the odd portion, theselected sensor electrodes are driven to the opposite voltages and thena negative demodulation is performed at Time G. Doing so may remove ormitigate the charge injection caused by the switches 320 coupling theselected sensor electrodes to the sensor line 330.

In one embodiment, the switches 320 add charge regardless whether thevoltage driven on the selected sensor electrodes is negative orpositive. By performing both positive and negative demodulation, thecharge added to the capacitive sensing measurement by the switches 320during the even portion is subtracted by the charge added to thecapacitive sensing measurement by the switches 320 during the oddportion. That is, when the results of performing the negative andpositive demodulation are combined, the charge injected by the switches320 in both of the even and odd portions in the CDM cycle cancels. Incontrast, because the selected sensor electrodes were driven to oppositevoltages, the measured charge on the sensor electrodes in the even andodd cycles combines to double the signal (i.e., the charge representingthe capacitive coupling between the sensor electrodes and the inputobject does not cancel but instead adds when the demodulation resultsare combined).

Although the CDM cycle in FIG. 11 is divided into an odd portion and aneven portion, the CDM cycle in FIG. 11 may result in capacitive sensingmeasurements with the same or better accuracy relative to the cycleshown in FIG. 9. In one embodiment, the two techniques described inFIGS. 9 and 11 can yield the same number of capacitive sensingmeasurement per unit of time. For example, the CDM cycle shown in FIG. 9may be repeated 100 times while the odd and even portions of a CDM cycleshown in FIG. 11 may each be repeated 50 times, thus yielding the samenumber of capacitive sensing measurements in the same block of time asthe technique illustrated in FIG. 9.

The offset sensor electrodes can perform a similar function in FIG. 11as these electrodes did in FIG. 9. That is, the switches 320 coupled tothe offset sensor electrodes are used to compensate for the chargeinjected by the switches 320 coupled to the selected sensor electrodes.Thus, in FIG. 11, using offset sensor electrodes and performing negativeand positive demodulation can be used in combination to mitigate thenegative effects of activating the switches 320 when coupling theselected sensor electrodes to the sense line 330. However, in anotherembodiment, the input device may not designate any of the sensorelectrodes in the columns as offset sensor electrodes. Put differently,performing negative and positive demodulation as shown in FIG. 11 may besufficient to mitigate the negative effects of charge injection, andthus, all the sensor electrodes in the column can be activelysensed—i.e., designated as selected sensor electrodes—in the CDM cycle.However, in other embodiments, the columns may include offset sensorelectrodes to further improve the signal measured by the sensor module204.

In one embodiment, when using the technique illustrated in FIG. 11, thereset switch may be only needed at the beginning of a burst or sensormodule 204 may not need the reset switch 405 though some form of DCfeedback may be used in its place, such as a resistor. That is, whendividing a CDM cycle into the even and odd portions using differentpolarity modulations, the sensor module 204 does not need to hold theoffset sensor electrodes to ground. As such, unlike in FIG. 9, FIG. 11does not include a RESET signal.

FIG. 12 illustrates a sensor module 1200 in accordance with anembodiment of the invention. Like the sensor module 204 illustrated inFIG. 3, the sensor module 1200 (or AFE) includes the op amp 410 with thefeedback capacitor and the reset switch 405 in parallel feedback pathsbetween the output of the op amp 410 and the inverting (−) input. Inaddition, the sensor module 1200 includes a measuring switch 1205 whichselectively couples the inverting input of the op amp 410 to the senseline 330 and a charging switch 1210 which selectively couples a driver1215 to the sense line 330.

When sampling the charge on the selected sensor electrodes, themeasuring switch 1205 is closed while the charging switch 1210 is open,thereby coupling the op amp 410 to the sense line 330 and the selectedsensor electrodes. However, when charging the selected sensor electrodesto the positive and negative voltages, the measuring switch 1205 is openwhile the charging switch 1210 is closed. For example, the driver 1215can drive the +/−7.5V onto the in selected sensor electrodes as shown atTimes B and C in FIG. 9 and Times A, B, E, and F in FIG. 11.

The advantage of adding the measuring switch 1205 and the chargingswitch 1210 to the AFE is that the drive lines 335, the drive selectlines 310, and the switches 325 can be removed from FIG. 3. That is,instead of using the drive lines 335 to charge the selected sensorelectrodes according to the voltage V_(tx), this function can beperformed using the charging switch 1210 and the driver 1215 shown inFIG. 12. Removing the switches 325 from the array can mean that that thedrive line 335 and drive select lines 310 may be removed and that thesensor area may be increased, resulting in larger signal. It alsoreduces the number of select lines by half.

FIG. 13 illustrates a sensor module 1300 in accordance with anembodiment of the invention. The sensor module 1300 (or AFE) includesthe reset switch 405, the feedback capacitor C_(FB), the op amp 410, andthe driver 1215 just like the sensor module 1200 in FIG. 12. However,the sensor module 1300 does not include the measuring switch 1205 andthe charging switch 1210 although the sensor module 1300 can operate ina similar manner as the sensor module 1200. That is, when sampling thecharge on the selected sensor electrodes, the driver 1215 is held to areference voltage so that the output of the op amp 410 generatescapacitive sensing measurements based on the total charge on theselected sensor electrodes coupled to the sense line 330. However, whencharging the selected sensor electrodes to the positive and negativevoltage, the driver 1215 can drive the +/−7.5V onto selected sensorelectrodes as shown at Times B and C in FIG. 9 and Times A, B, E, and Fin FIG. 11.

The advantage of the sensor module 1300 relative to the sensor module1200 is the reduced cost and complexity by removing the measuring switch1205 and the charging switch 1210. However, the voltage swing outputtedby the driver 1215 to charge the selected sensor electrodes may belimited relative to using the sensor module 1200. Regardless of whetherthe sensor module 1200 is used or the sensor module 1300, the drivelines 335, the drive select lines 310, and the switches 325 can beomitted from the sensor layout 300.

In another embodiment, a third switch (e.g., a TFT transistor) can becoupled to each of the sensor electrodes 315 shown in FIG. 3. The otherend of the third switch can be coupled to a second drive line(independent of the drive lines 335). Moreover, the gate of the thirdswitches can be controlled by second drive select lines (independent ofthe drive select lines 305). The advantage of adding these components isthat the input device can use one set of drive lines to drive thepositive voltage V_(tx) onto the selected sensor electrodes and theother set of drive lines to drive the negative voltage V_(tx) onto theselected sensor electrodes. Because in this embodiment the voltages onthe drive lines do not change, the reset switch 405 in the sensor modulemay not be needed or needed less frequently resulting in fasteroperation. In one embodiment, the reset switch 405 is replaced by aresistor or other circuit element which provides DC feedback.

The embodiments and examples set forth herein were presented in order tobest explain the embodiments in accordance with the present technologyand its particular application and to thereby enable those skilled inthe art to make and use the present technology. However, those skilledin the art will recognize that the foregoing description and exampleshave been presented for the purposes of illustration and example only.The description as set forth is not intended to be exhaustive or tolimit the disclosure to the precise form disclosed.

In view of the foregoing, the scope of the present disclosure isdetermined by the claims that follow.

We claim:
 1. A processing system for operating a plurality of sensorelectrodes disposed in an array, wherein a first sensor electrode and asecond sensor electrode of the plurality of sensor electrodes aredisposed in a same column in the array, the processing systemcomprising: a controller circuit configured to: charge the first sensorelectrode to a first voltage; charge the second sensor electrode to asecond voltage different from the first voltage, wherein the first andsecond voltages are assigned to the first and second sensor electrodesaccording to a code; and after charging the first and second sensorelectrodes, couple the first and second sensor electrodes simultaneouslyto a same analog front end (AFE) to sense a combined charge on the firstand second sensor electrodes.
 2. The processing system of claim 1,wherein the controller circuit is configured to: repeatedly charge thefirst and second sensor electrodes according to the code and generatemultiple measurements of the combined charge using the same AFE, whereinthe processing system is configured to determine a first individualchange in capacitance corresponding to the first sensor electrode and asecond individual change in capacitance corresponding to the secondsensor electrode using the multiple measurements of the combined charge.3. The processing system of claim 2, wherein the code comprises an Nnumber of combinations of driving the plurality of sensor electrodes todetermine individual charges stored on an N number of the plurality ofsensor electrodes.
 4. The processing system of claim 1, wherein thefirst voltage is a positive voltage relative to a reference voltage andthe second voltage is a negative voltage relative to the referencevoltage.
 5. The processing system of claim 4, wherein after sensing thecombined charge of the first and second sensor electrodes, thecontroller circuit is configured to: charge the first sensor electrodeto the second voltage according to the code; charge the second sensorelectrode to the first voltage according to the code; and after chargingthe first and second sensor electrodes to the second and first voltages,respectively, couple the first and second sensor electrodessimultaneously to the same AFE to sense a different combined charge onthe first and second sensor electrodes.
 6. The processing system ofclaim 1, wherein the controller circuit is configured to operate afingerprint sensor comprising the plurality of sensor electrodes.
 7. Aninput device, comprising: a plurality of sensor electrodes disposed incolumns forming an array, wherein each of the columns comprises a senseline coupled to an AFE and respective transistors selectively couplingsensor electrodes in the column to the sense line; a processing systemconfigured to: charge at least one of the plurality of sensor electrodesin at least one of the columns to a first voltage; and after chargingthe at least one of the plurality of sensor electrodes: couple the atleast one of the plurality of sensor electrodes to the sense line of theat least one of the columns by activating one of the respectivetransistors; deactivate an offset transistor coupled to the sense lineto mitigate charge injected by activating the one of the respectivetransistors; and measure a charge on the at least one of the pluralityof sensor electrodes using the AFE after deactivating the offsettransistor.
 8. The input device of claim 7, wherein the processingsystem is configured to: charge multiple sensor electrodes in the atleast one of the columns to different voltages; and after charging themultiple sensor electrodes: couple the multiple sensor electrodes to thesense line by activating a plurality of the respective transistors,wherein deactivating the offset transistor is performed after chargingthe multiple sensor electrodes; and measure a combined charge on themultiple sensor electrodes in parallel using the AFE after deactivatingthe offset transistor.
 9. The input device of claim 8, wherein thedifferent voltages are selected according to a code, wherein the codecomprises multiple combinations of driving voltages onto the sensorelectrodes in the at least one of the columns during multiple cycles.10. The input device of claim 9, wherein the processing system isconfigured to identify an individual change in capacitance for one ofthe sensor electrodes in the at least one of the columns from multiplecombined charge measurements obtained during the multiple cycles. 11.The input device of claim 7, wherein the offset transistor is coupled toan offset sensor electrode in the at least one of the columns, whereinthe offset sensor electrode is disconnected from the sense line when thecharge is measured.
 12. The input device of claim 7, wherein the offsettransistor is a dedicated offset transistor and is not directly coupledto any sensor electrode.
 13. The input device of claim 7, wherein theoffset transistor is coupled to an offset sensor electrode in the atleast one of the columns, wherein the offset transistor is activatedwhile charging the at least one of the plurality of sensor electrodessuch that the offset sensor electrode is coupled to the AFE.
 14. Theinput device of claim 13, wherein the processing system is configuredto: before coupling the at least one of the plurality of sensorelectrodes to the sense line, deactivate a reset switch in the AFE; andbefore measuring the charge on the at least one of the plurality ofsensor electrodes, measure a charge on the offset sensor electrode usingthe AFE to compensate for charge injected by the reset switch.
 15. Aprocessing system for operating a plurality of sensor electrodesdisposed in columns forming an array, wherein each of the columnscomprises a sense line coupled to an AFE and respective transistorsselectively coupling sensor electrodes in the column to the sense line,the processing system comprising: a controller configured to: charge atleast one of the plurality of sensor electrodes in at least one of thecolumns to a first voltage; and after charging the at least one of theplurality of sensor electrodes: couple the at least one of the pluralityof sensor electrodes to the sense line by activating one of therespective transistors; and deactivate an offset transistor coupled tothe sense line to mitigate charge injected by activating the one of therespective transistors; and a sensor module comprising the AFE, whereinthe sensor module is configured to measure a charge on the at least oneof the plurality of sensor electrodes using the AFE after the offsettransistor is deactivated.
 16. The processing system of claim 15,wherein the controller is configured to: charge multiple sensorelectrodes in the at least one of the columns to different voltages; andafter charging the multiple sensor electrodes: couple the multiplesensor electrodes to the sense line by activating a plurality of therespective transistors, wherein deactivating the offset transistor isperformed after charging the multiple sensor electrodes; and measure acombined charge on the multiple sensor electrodes in parallel using theAFE after deactivating the offset transistor.
 17. The processing systemof claim 16, wherein the different voltages are selected according to acode, wherein the code comprises multiple combinations of drivingvoltages onto the sensor electrodes in the at least one of the columnsduring multiple cycles.
 18. The processing system of claim 15, whereinthe offset transistor is coupled to an offset sensor electrode in the atleast one of the columns, wherein the offset sensor electrode isdisconnected from the sense line when the charge is measured.
 19. Theprocessing system of claim 15, wherein the offset transistor is adedicated offset transistor and is not directly coupled to any sensorelectrode.
 20. An input device, comprising: a plurality of sensorelectrodes disposed in columns forming an array, wherein each of thecolumns comprises a sense line coupled to an AFE and respectivetransistors selectively coupling sensor electrodes in the column to thesense line; wherein the AFE is configured to: charge a first sensorelectrode in at least one of the columns to a first voltage; charge asecond sensor electrode in the at least one of the columns to a secondvoltage different from the first voltage; and after charging the firstand second sensor electrodes, measure a combined charge on the first andsecond sensor electrodes in parallel.
 21. The input device of claim 20,wherein the AFE comprises a first switch selectively coupling anoperational amplifier to the sense line and a second switch selectivelycoupling a driver to the sense line, wherein the first switch is openand the second switch is closed when charging the first and secondsensor electrodes and the first switch is closed and the second switchis open when measuring the combined charge.
 22. The input device ofclaim 20, wherein the AFE comprises a driver coupled to a first input ofan operational amplifier while the sense line is coupled to a secondinput of the operational amplifier, wherein the driver is configured tocharge the first and second sensor electrodes to the first and secondvoltages using the operational amplifier and the sense line.